[MUSIC] November 30th 1954 was a bad day for Mrs. Ann Hodges of Sylacauga, Alabama.

She was taking a nice nap on her couch when a grapefruit-sized rock crashed through the ceiling, busted the radio, and slammed right into her.

She survived, but was left with a heckuva bruise... and the title of "only known person to be hit by a meteorite" These things obviously don't happen often.

Compared to death by space rock, you've got a better chance of getting struck by lightning while...

I dunno, watching the Cubs win the World Series?

Cubs win the World Series.

Okay Maybe this is more serious than I thought.

[MUSIC] One look at the moon and it's clear the solar system is a violent place.

But Earth's weather and tectonic activity have erased most of our planet's impact scars... has that given us a false sense of safety?

Two hundred years ago, people were just realizing that fossils buried in the Earth belonged to creatures that don't exist anymore, that species could actually go extinct.

Looking through these geologic obituaries, we know that around 66 million years ago, a lot of them went extinct at once, when a 10 km-wide asteroid landed in what is today Mexico's Yucatan Peninsula.

Although when I say "landed" I really mean that a small mountain, traveling 20 km per second and glowing brighter than the sun, collided with enough force to instantly melt the crust, eject it out of the atmosphere, and shower the Earth with a literal fiery rain of death.

If a few kilometers wide doesn't sound that big, it isn't.

Look at the size of that dino-killer asteroid next to our planet.

Can't see it?

It's right here.

At asteroid impact speeds, kinetic energy is cruel.

Now mass extinctions take millions of years to play out, but one very bad day and one unlucky rock can be enough to set one off.

The next big impact?

It's a matter of when, not if.

After the planets formed, the solar system was left with a bunch of leftover rocky stuff.

Anything bigger than one meter across we call an asteroid.

Smaller bits, from a meter down to a millionth of a meter, we call meteoroids.

Forty to a hundred tons of "stuff" enters Earth's atmosphere every day... but luckily most of it's smaller than a grain of sand.

When these small meteoroids burn up on entry they're called meteors.

And any bit that makes it to Earth's surface becomes a meteorite, like the rock that woke Ann Hodges up from her nap.

But none of this little stuff is "end of species" material.

We think the solar system is home to more than a million asteroids over a kilometer wide, and most of them live in the "asteroid belt" between Mars and Jupiter, where they don't threaten Earth, which is nice if you remember that whole dead dinosaur/fiery rain of death thing.

But more than 10,000 asteroids of various sizes have orbits that cross Earth's.

What are the chances that one of these will hit us?

By studying craters on the moon, we can estimate how often different-sized impacts tend to happen.

It's likely that a Hiroshima-level meteor or asteroid explosion happens once a year on Earth, but so much of the planet is uninhabited or covered in water that we never see or hear them.

20-meter asteroids, the same size as the one that exploded over Russia in 2013, happen about twice a century, and usually break up before hitting Earth.

100-meter asteroids, big enough to destroy a large city and about twice the size of what created Arizona's Barringer Crater happen about every 5,000 years.

A 1 km asteroid strikes Earth about every half million years, and anything larger could put enough dust into the atmosphere to cause long-term climate change ... nuclear winter, asteroid edition.

There's about a thousand asteroids this size whose orbits cross ours.

Estimating an asteroid's damage is difficult.

The angle of impact, what it's made of, whether it strikes water or land... can all change the outcome.

Take relative velocity.

Two cars colliding head-on will impact with more energy than those same two cars in a rear-end collision.

Earth is traveling around the sun at 30 km/second, enough to move the entire width of our planet every seven minutes, so depending on if an asteroid or meteoroid encounters Earth from the rear or head-on, it could enter the atmosphere anywhere from 11 to 72 km/second, a big difference in the end... or not the end.

If you want to see for yourself how these variables can change the impact, check out the Impact Earth!

simulator, it's a cool web tool that lets you plug in different impact scenarios and pummel Earth to your heart's content without any danger of actual apocalypse.

When it comes to mass extinction-level asteroids, we don't have much data.

Last time one hit Earth was 66 million years ago.

The bigger the asteroid, the more rare and the more infrequent the impact, but asteroids don't operate on a train schedule, theoretically an extinction level impact could happen at any moment.

The good news is we've probably found more than 90 percent of the big asteroids near Earth, but the bad news is we haven't found the other ten percent.

Scientists have put all known orbits into big science machines, and there's no object that we know of with a serious chance of hitting Earth in the near future.

But that doesn't mean one's not out there.

It also doesn't mean that one is out there.

If we do find one headed our way?

First, freak out a little.

That's ok.

It's only natural.

But then we'd have to take a deep breath and get back to science.

We'd need to push it off course, just a tiiiny little bit.

Really won't take much to avoid impact, no need to go all Bruce Willis on it.

We could fire a rocket and hit it, change its speed a little, but that might break one deadly asteroid into dozens of small deadly asteroids.

My favorite idea is that we could put a spacecraft right next to it, and let the mutual gravity nudge it eeever so slightly, like a gravity tug boat.

We could even paint one side of an asteroid, and change how many photons it absorbs or reflects, use /actual sunlight/ to nudge it off course.

Of course, all of these options depend on having a robust space program, something the dinosaurs didn't have.

So unless we want to end up like them... Go NASA!

The asteroid that killed most of the dinosaurs made it possible for our lineage to arise.

If another space rock pulls the plug on the human experiment, I'm sure another species will take our place, but I don't really want to turn the planet over to the cockroaches just yet.

Instead, we can be the first species that's ever had a say in their own future, the first species that could make an impact by avoiding an impact.

Stay curious... and sleep tight!